Pundit
Post by Gryffles on Apr 5, 2017 22:59:44 GMT 12
Now that I have my system up and running and I can enjoy spinning vinyl again I'd like to try a SUT in the future. The Walker phono stage is adjustable for gain and load by changing resistors internally so thats helpful. My cartridge is a Benz Micro Wood SL which is .4mv output and 12ohm impedance. So I'm thinking I would lower the gain of the phono stage to approx 40dB and change the load to 47k ie standard MM After doing a bit of reading and using one of the handy online calculators available link I came up with the following scenario (please feel free to point out anything I may have missed or should consider etc) MC Output .4mv Turns Ratio 1:15 = 23.5dB gain =6mV into the MM input of phono stage Cartridge would see a load of 208ohm Now the load should probably be closer to 270ohm so I would then need to change the phono load resistor to 62k to load the cartridge to 275ohm From what I understand the above should tick all the boxes for a good match. One thing I don't understand is the relationship between the primary winding impedance of the transformer and the cart impedance. Is the perfect relationship for my cartridge a transformer which has 15ohm input impedance?
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Post by Owen Y on Apr 6, 2017 10:10:09 GMT 12
Hi Gryffles - I think you have it it pretty much spot on. The Benz specif is for >100 ohms load for the SL, so a 1:15 SUT would do the trick (208ohms load as you say). However, because of the flexibilty of your phonostage, you can use other SUT ratios (eg if you can't get a good 1:15 one) & simply adjust the 47kohm input R to get the loading you desire. Also, depending on your preamp-amp gain/sensitivity, you may or may not want more phonostage output - eg a higher gain SUT may or may not help. Additionally, the phonostage adjustability allows you to test cart loading sonically, to suit you.
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Post by Owen Y on Apr 6, 2017 11:29:53 GMT 12
Have you tried the Siemens T42 with the Walker & Benz SL?
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Pundit
Post by Gryffles on Apr 6, 2017 21:16:27 GMT 12
Hi @owen Y
Thanks for the reply and confirmation I'm on the right track.
I haven't tried the Siemens T42 although I will once I order some 47k resistors for the phono stage. I have plenty of gain now so the 20:1 of the Siemens will not be a great match and will put the output into the MM input over 10mv.
I'm keeping my eyes open on Ebay for something more suitable though.
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Post by Owen Y on Apr 7, 2017 13:36:36 GMT 12
Seldom is 'too much gain' a problem, esp if you are using a passive vol control/linestage in front of your amp - you can always attenuate it. But lack of gain is a sonic killer  Some Qs.... 1) The Walker Phono is a MM/MC, 40-66dB phonostage, if I understand correctly? So the Benz Wood SL 0.4mV could be amplified 2000x to 800mV - which would be OK with a normal 20dB linestage + amp, but may be marginal/unsuitable for a passive linestage/an insensitive amp. 2) Did the Walker phono not come standard with 47k ohm input Rs? If so, the T42s should work. 3) Traditionally, a low output MC is coupled with an SUT of 20-28dB - which provides an input loading for the cartridge of 470-50 ohms. So, I wouldn't be concerned about a 1;20 SUT (26dB), as the resultant ~70 ohms loading is not necessarily unsuitable for the Benz SL - besides, you have the flexibility of increasing the input Rs in the Walker to whatever suits your setup - eg. this my line-up of MC cart loading plugs (with some shorting plugs)..... 
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Pundit
Post by Gryffles on Apr 7, 2017 21:43:37 GMT 12
The Walker is MM/MC, 40 to 66dB gain
It did come with 47k resistors however one leg has broken off one. I'll have to have a good look to see if I can repair it but they are Vishay Z Foil so rather delicate. Failing that I'll order some replacements but at close to $20 USD each + freight they are not cheap.
I will give the Siemens SUT a go tomorrow if I can repair the resistor and see how I get on.
My amp is very sensitive so gain has never been a problem but I hear what you are saying in regards to this.
Will let you know how I get on tomorrow.
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Post by Owen Y on Apr 8, 2017 12:16:05 GMT 12
I use Shinkoh Tantalums, in these sort of low signal positions - if the cost (~NZ$5 ea) is acceptable. (Personally I've never liked the sound of Vishays. Maybe just me.)
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Pundit
Post by Gryffles on Apr 15, 2017 20:43:05 GMT 12
I finally got around to trying the Siemens T42 SUT today. Albeit with el-cheapo Jaycar metal film load resistors in the phono stage.
Got to say I'm impressed with the results despite a hum problem and to much gain.
I've tried every possible grounding combo which results in a reduction in hum but it's still there. After looking online it appears that a couple of wires in my SUT that aren't connected at all are connected to -ve in other peoples builds. I'll try this tomorrow and see what happens.
This SUT business seems to be a sound that I like.
Now to find 1:15 transformers to lower the gain.
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Post by Owen Y on Apr 16, 2017 21:09:49 GMT 12
Be sure you have the -ve on ea side of the SUT connected through, as per the JE Labs diagram below: 
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Pundit
Post by Gryffles on Apr 16, 2017 22:25:09 GMT 12
Both -ve are joined as above however the SUT chassis ground post is connected to both -ve so that may be causing a ground loop. Will play around tomorrow.
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Post by Owen Y on Apr 17, 2017 10:06:24 GMT 12
Yes, ea ch should be connected as above - in ea individual channel. The transformer encasings (steel or mumetal) should be connected to one Gnd point - either the box (if they are mounted to/onto a box) or to the -ve. The transformer core may have a 'screen' (shown asterisked in diagram) should be Gnded also - but with SUTs, it sometimes pays to experiment. Ans yes, avoid any Ground 'loops' 
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Post by Owen Y on Apr 17, 2017 11:20:38 GMT 12
Gryffles - I'm still surprised that you have too much gain, using a Benz Wood SL & a passive (Prometheus) TVC. Granted the Bastanis could be 100dB/W sensitive, & you'd need only maybe only 1/50 of a Watt from your 2A3 SETs for LOUD listening at say 3.6m distance.... so the TVC would be attenuated around 36dB (48 times) - which should be OK. For comparison, my spkrs are VERY sensitive (105db/W) & my TVC is down at 3-5 clicks from the bottom (42-36dB attenuation), which is just OK, but the lower vol settings are not ideal because of the larger dB steps of the 'Log curve'. It is likely that the Prometheus is also set up to mimic a Log Curve, which means that the lower vol steps are coarse, becoming finer/closer further up. So, if you are attenuated say ~36 dB or more for normal listening, you may be experiencing large volume steps with no fine vol adjustment. How many steps up from max attenuation are you for normal listening?
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Pundit
Post by Gryffles on Apr 17, 2017 15:15:45 GMT 12
Owen YYeah I didn't understand why I have so much gain either. Looks like there is some conflicting info on these transformers. Mine are T42/75/1 which according to a chap who uses these in HK produce the following: T42/75/1 what we have - used in mobile cinema amplifier Siemens. DCR input 3,5 Ohm, DCR output 3 kOhm. T42/75/1 are the best in this family, much better as the T42 or T42/2. Spec: Load resistor: 10 kOhm – ratio 1:25 - amplification rate 27,9 dB Load resistor: 22 kOhm – ratio 1:30 - amplification rate 29,5 dB Load resistor: 47 kOhm – ratio 1:40 - amplification rate 32,5 dB Without any load resistor – ratio 1:50 - amplification rate 34 dB So I was looking at the Tube Classics site which refers to the T42 transformer which is different. Seems there a number of differing versions of the T42. Sorry for the confusion. At least I know why I have to much gain now
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Post by Owen Y on Apr 17, 2017 16:04:21 GMT 12
What vol setting do you normally use, steps up from the bott., max attenuation?
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Pundit
Post by Gryffles on Apr 17, 2017 16:43:56 GMT 12
Normally I would have the pre at 2/3rd of the way round. Haven't counted the steps yet.
I also have a vol pot on the 2A3 amp which is usually set at 2/3rd.
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Post by Owen Y on Apr 17, 2017 18:37:20 GMT 12
Vol at 2/3 is quite high. 1/3 to 1/2 vol is better - because you thus have lots of headroom in reserve - at least 20dB from max volume is good (at least - 30dB even). Most amps have no vol attenuator. It is better for the amp to run out of puff, but not the preamp. But your system should be in no danger of either. What is the general amp cct?
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Pundit
Post by Gryffles on Apr 20, 2017 21:03:19 GMT 12
2/3 would be the max Owen.
I have measured listening levels and they are usually around 80dB average.
The amp is a one off circuit and always had a lot of gain but sounded good so it stayed. Was built by a chap in Akl.
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Post by Owen Y on Apr 20, 2017 22:07:39 GMT 12
Ah that's good, 80dB av SPL is quite typical 'loud-ish' listening level (for rock/pop/classical, highish for Jazz). What tubes doe the 2A3SET amp use?
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Pundit
Post by Gryffles on Apr 22, 2017 0:20:15 GMT 12
It uses 6c45 and 6l6
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Post by Graham on Jun 21, 2017 9:27:25 GMT 12
I have also dipped my toes into the mysterious world of Step Up Transformers recently as I kept reading that this was a better option than using the MC stage on my preamp. However a quick bit of googling resulting in the prices making me nearly fall off my perch ! In fact I have ordered the bits required to DIY a highly recommended Boozehound Pre Preamp kit, but that's another story. BUT - last week a vintage Ortofon T-10 popped up on trade Me which looked very interesting. A bit of homework showed that this was designed by Ortofon for their MC100 series cartridges with very low output of 0.09 - 0.1mV and 3 ohms output impedance. This was similar to my little Excel Pro 81 output of 0.2mv and 3 ohms impedance. Further homework confirmed that the T-10 had a gain of 32dB which combined with the ProJects 40dB MM stage gave me an impressive total of 72dB. The 60dB of the MC stage had proven to be not quite enough resulting in having to use 1/2 to 3/4 volume. Looked more and more promising. To cut a long story short, I bought ( stole ) the little sucker for $100 and it works brilliantly !!!! So much better sound stage with clear identification and placement of individual instruments and singers. Better high frequency extension with no harshness or shillness. Female voices sound lovely. I realise this T-10 is fairly near the bottom of the SUT foodchain in hi fi terms but it is a very impressive little device and matches my one and only LOMC cartridge so well. My only tiny criticism is it probably has a bit too much gain for my system as I now have the volume control at only 1/4 to 1/2 position. But I can live with that. The moral of the story is that it is vital to match up SUT specs to that of your cartridge. Do you homework and you will be rewarded. If you don't, even the most exotic SUT could sound disappointing in your system. It's got me wondering what SUTs further up the food chain might sound like, but then I would probably want to upgrade the cartridge also ! Oh dear, it's never ending
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Post by Owen Y on Jun 21, 2017 10:00:45 GMT 12
That's excellent news, Graham. We should post up some basic info on SUTs - although seemingly mysterious, there are only a couple of parameters to consider in selection. Also, without re-inventing the wheel every time, if we look back, we'll find that low-med output MC cart's were traditionally used with 26-28dB (20-25x gain) SUTs. The Ortofon T-10 is 32dB (40x) & was designed for Ortofon's v low output MC cart's of the early 80s, eg MC30. Plugged into the 47kohm phono input, Graham's Excel Pro 81 will be seeing a load of only 47000/1600 = 30 ohms. However, the Pro 81 coil imped is only 3 ohms - a 1:10 source/load ratio, which is in the desired range.
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